The use of small, particulate, agglomerates of relatively fine abrasive grits held in a matrix, for use as a substitute for conventional abrasive grits on a coated abrasive ("sandpaper") flexible abrasive, was suggested at least as early as the Jackson U.S. Pat. No. 2,194,472. So far as is known, the solid agglomerates of the type disclosed in the Jackson patent or products made from them have never been commercially successful. U.S. Pat. No. Re. 29,808, to Wagner, discloses hollow spheres (or other shapes, such as cylinders) consisting of abrasive grits bonded onto the outer surface of a friable matrix, such as resin or an inorganic silicate. Kressner copending application Ser. No. 06/129,186, now U.S. Pat. No. 4,311,489, filed Mar. 10, 1980, discloses solid agglomerates bonded by fused cryolite or other "salts or silicates". European published application No. 8868, published Mar. 19, 1980 is similar, and is based on the parent application of U.S. Ser. No. 06/129,186, now U.S. Pat. No. 4,311,489.
British Pat. No. 982,215 and U.S. Pat. No. 3,156,545, teach making solid agglomerates for use in grinding wheels consisting of glass bonded alumina or other grits. Benner U.S. Pat. No. 2,216,728 discloses glass or metal bonds for the matrix of aggregates containing diamond abrasive particles. The patent states that the matrix may be made somewhat porous to enhance mechanical bonding when the aggregates are mixed with a binder to form a grinding wheel, referring obviously to open porosity. Robie Pat. No. 2,806,772 suggests including foamed glass in abrasive agglomerates bonded by a resin matrix.
While abrasive agglomerates of the hollow resin or silicate bonded type have shown good results in coated abrasive applications, and agglomerates such as taught in the Kressner application show good results, both types of agglomerates are difficult or expensive to manufacture and it is difficult to control their friability.